1. Field of the Invention
The present invention relates to a plasma CVD apparatus for forming a thin film in the semiconductor, liquid crystal, optical disc or like technological fields, and particularly relates to a plasma CVD apparatus using a reaction chamber constituted by a conductor such as metal.
2. Description of the Related Art
As methods of forming a thin film on a substrate, there are known a sputtering method using a sputtering phenomenon in a decompressed state, a vacuum evaporation method using an evaporation phenomenon, a CVD (Chemical Vapor Deposition) method such as a plasma CVD method using low temperature gas decomposition by plasma, a thermal CVD method using heat decomposition of a gas, and a photo CVD method for decomposing a gas by energy of shortwave light or ultraviolet rays, and the like. In addition, research and development has been undertaken with respect to combined techniques and applied techniques of the aforementioned methods, and such techniques have been implemented in actual manufacturing methods.
Among the foregoing thin film forming techniques, the plasma CVD method is characterized in that direct current or high frequency voltage is applied to a reaction gas in a decompressed state, and the reaction gas is decomposed by glow discharge to deposit a film on a substrate. In the thin film formation by this method, gas can be decomposed at a relatively low temperature (500.degree. C. or less) by plasma energy such as high electron temperature of several eV in the plasma, and films of various compositions with high purity can be formed by using vacuum and by changing the kind of gas. Thus, the plasma CVD method is used in various fields such as the semiconductor field, the liquid crystal field, the optical disk field, and the magnetic disk field. magnetic disk field.
It is well known that to use a batch type plasma CVD apparatus in which a plurality of substrates are processed at the same time to form a thin film on a substrate.
However, in the case of the batch process, even if substrates are processed at the same time, characteristics of thin films slightly fluctuate in the respective substrates. Thus, repeated or consistent precision is poor and unevenness among substrates is large, so that the batch process has not been able to fulfill the desire for high precision in a thin film.
In addition, since a plurality of (about four to eight pieces) substrates are processed at the same time, it has been necessary to provide a substrate holder on which the substrates are mounted and are moved together with the substrates. This substrate holder is removed to the outside of the plasma CVD apparatus when film growth on the substrates is completed, the next batch of substrates is mounted on the substrate holder, and then the substrate holder is again processed in the apparatus.
Thus, since the process of heating in vacuum and placing in atmospheric pressure at room temperature is repeated, a so-called peeling phenomenon occurs in which a film attached to the substrate holder is peeled off.
Because of the foregoing reason, the batch process has not been used recently in not only the plasma CVD apparatus but also in almost entire fields including, for example, thin film etching. Instead, single wafer processing type apparatus has been used.
The single wafer processing type is a system characterized in that a substrate holder moving together with substrates is not used, but, rather, substrates are processed one by one, and only the substrate is moved. A conventional plasma CVD apparatus using this system will be described with reference to FIGS. 2 and 3.
FIG. 2 is a top view showing a single wafer processing type plasma CVD apparatus, and a chamber 201 is a load chamber in and out of which a substrate is carried. Chambers 202 to 206 become reaction chambers.
A plurality of substrates to be processed are set in the load chamber 201 by a cassette or the like. After the substrates are set in the load chamber 201, the chamber is decompressed. When the chamber is decompressed to a sufficient pressure, a gate valve 210 between the load chamber 201 and a common chamber 207 is opened. A substrate carrying means 208 disposed in the common chamber 207 carries one substrate among a plurality of substrates set in the cassette in the load chamber 201 from the load chamber 201 into the common chamber 207. FIG. 2 shows the state where the substrate has been carried, and a substrate 209 is carried into the reaction chamber in which a thin film is formed. The substrate 209 is carried by the substrate carrying means 208 into the reaction chamber.
The common chamber 207 is connected to the respective reaction chambers 202 to 206 and the load chamber 201 through the respective gate valves 210. When the substrate 209 is carried in and out of the respective chambers, the gate valve of that chamber is opened. The load chamber 201, the respective reaction chambers 202 to 206, and the common chamber 207 are evacuated by vacuum exhausting means, respectively.
As to thin film formation, there are various types such as a lamination type (P-layer, I-layer, and N-layer, etc.)for use, for example, in an amorphous solar cell, and a single layer type for use, for example, in a protective film for a semiconductor. Thus, the processes in the respective chambers are different according to the kinds of films to be formed, the type of lamination, and the like.
FIG. 3 is a sectional view taken along line A--A of FIG. 2 and showing the common chamber 207 and the reaction chamber 204.
An electrode 211 and a substrate holder 212 are disposed in the reaction chamber 204. The electrode 211 is connected to a power source 213, and the substrate holder 212 and the reaction chamber 204 are grounded. The substrate holder 212 is equipped with a heater (not shown) for heating a substrate.
This substrate holder 212 is disposed inside the reaction chamber 204 contrary to the foregoing batch type, and is not moved together with the substrate 209.
The substrate 209 is placed on the substrate holder 212 from the common chamber 207, and a reaction gas is introduced through an introduction pipe 214. Then voltage is applied to the electrode 211 to generate plasma in a space 215 so that a thin film is formed on the substrate.
The substrate 210 on which a thin film has been formed, is again carried by the carrying means 208 in the common chamber 207 from the reaction chamber 204 into the common chamber 207, and is subjected to a next process. Another substrate is carried in the reaction chamber 204 and thin film formation is performed in the same manner. In these sequential processes, only the substrate is moved.
Incidentally, reference numerals 217 and 218 denote vacuum exhausting means, which maintain the inside of the common chamber and the reaction chamber in a decompressed state. The exhausting means is generally independently provided in the respective chambers.
The reaction cambers 203 to 206 also have the same structure as the reaction chamber 202, and the reaction chambers are selectively used according to the kind and thickness of a film to be formed. For example, a silicon film is formed in the reaction chamber 202, a silicon oxide film is formed in the reaction chamber 203, and a silicon nitride film is formed in the reaction chamber 204.
Alternatively, the same process of laminating a silicon nitride film, a silicon film, and a silicon nitride film is performed in the respective reaction chambers, so that the total throughput, that is, the so-called producibility is improved.
Of course, if the kind of film to be formed in each chamber is determined so as to suppress impurities to the highest degree, each film can be sequentially formed without mixture of impurities, so that it is also possible to increase the efficiency of production.
In the structure of the above-mentioned plasma CVD apparatus, the respective chambers are mainly composed of a conductor such as metal, for example, aluminum or stainless steel. With respect to materials for a chamber of the plasma CVD apparatus, although it is known that quartz or alumina as an insulator may be used other than metal, such a material is not used for a single wafer processing type apparatus. The reason is as follows.
In the case of the single wafer processing type plasma CVD apparatus, since substrates are processed one at a time, it is necessary to provide a plurality of reaction chambers to increase the producibility. If a plurality of reaction chambers are provided, a plasma CVD apparatus inevitably becomes large. Thus, it is necessary to use a material having strength. In the case of a material such as quartz or aluminum, although it has strength, it is apt to be damaged. A material of a vacuum chamber is so subtle that even if a flaw such as a hairline crack is present, vacuum can not be maintained. In addition, since the apparatus becomes large and complicated, it is necessary to use a material which is easily workable and has high working precision. Moreover, it is preferable that the material be as inexpensive as possible.
To satisfy the above described conditions reaction chamber, presently are often made of a metal material such as aluminum, aluminum alloy, or stainless steel.
In the case where a thin film is formed by the foregoing single wafer processing type plasma CVD apparatus, a reaction gas to be decomposed extends not only to a substrate but also to the entirety of a reaction chamber. In a thermal CVD and the like, since the entirety of a reaction chamber is heated, films are formed over the entirety of the reaction chamber, as well as on the substrate. In the case of the plasma CVD method, although it is ideal that a film is formed only on the substrate where plasma is generated, films are also formed at places other than the substrate. That is, since the plasma 215 extends also in the space other than the vicinity of the substrate 209, films are formed also on the exposed portions such as the surface of the electrode 211 and the inner wall of the reaction chamber.
The aforementioned occurs because, since the reaction chamber is made of a metal material, that is, a conductor, plasma is not generated only in a space between the electrode and the substrate holder, but extends beyond this space. Contrary to the substrate holder of a batch type plasma CVD apparatus, the films formed on portions other than the substrate are not exposed to the atmosphere, or are not subjected to the repetition of a cycle of room temperature and high temperature, so that the films do not immediately peel off.
However, when film formation is continued, the films start to peel off as well. Then these films become particles, flakes or the like and fall onto the substrate or the bottom of the reaction chamber.
Thus, it is necessary to periodically remove the films formed and deposited on places other than the substrate after several sequences of film formation are carried out and before the films start to peel off. The removal of the films is carried out by introducing an etching gas into the reaction chamber to form plasma so that the films are etched.
At the film formation, there is also a terrible case such that unnecessary discharge such as arc discharge occurs also in a space near the electrode 211 and between the electrode 211 and the inner wall of the reaction chamber, for example, in the space designated by 216 in FIG. 3, and the thickness of a film formed on the inner wall becomes thick, so that the film becomes easy to peel off.
The inner wall of the reaction chamber is made relatively smooth in its surface. The objects thereof are to suppress the degassing of the wall so that impurities are decreased, as well as to prevent arc discharge, and the like. Actually, the wall surface is made to be close to a mirror surface by buff polishing with #400 or more, electrolytic polishing, combined electrolytic polishing, or the like. A film attached to the smooth surface has poor adhesion and easily peels off. The peeled film becomes particles or flakes, falls onto the reaction chamber, and is deposited. These deposits, which once peeled off and became particles or flakes, are hard to remove by plasma etching than a film-like substance attached to the inner wall at the film formation.
Actually, the deposits can not be completely removed. The reason why they can not be completely removed, is not clarified theoretically. However, as an empirical law, although a film-like substance may be etched, a solid substance such as particles or flakes can not be completely etched.
Thus, it is necessary to carry out cleaning of the reaction chamber considerably before a film that is attached to a place other than a substrate peels off. Thus, with respect to the ratio of film forming hours contributing to production to etching hours not contributing to production in hours of operation of an apparatus, the ratio of the hours contributing to production is decreased.
If unevenness is provided on the surface of the reaction chamber, it is possible to prevent the film from easily peeling off. However, with a surface area that becomes large, it takes a more time to achieve evacuation, and the amount of gas released from the uneven surface having the increased surface area is increased. Thus, the method of providing the unevenness is contrary to the original object to form a thin film having high purity by using a vacuum apparatus.